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See Also: Fermentation (biochemistry)



Anaerobic respiration refers to the Oxidation of molecules in the absence of Oxygen to produce energy, in opposition to Aerobic Respiration which does use oxygen. Anaerobic respiration processes require another Electron acceptor to replace oxygen. Anaerobic respiration is often used interchangeably with Fermentation , especially when the Glycolytic Pathway is used for energy production in the cell. They are not synonymous terms, however, since certain anaerobic Prokaryote s can generate all of their ATP using an electron transport system and ATP synthase.
General word and symbol equations for the anaerobic respiration of glucose can be shown as

:
'' Glucose o Lactic Acid + energy ( ATP );''


:
''C6H12O6 o 2C3H6O3 + 2 ATP.''


The energy released is about 120 kJ per mole glucose.


Obligate anaerobes

In some organisms called '''' (causes Tetanus ), '' Clostridium Perfringens '' (causes Gangrene )), the presence of oxygen is lethal. This is because the presence of oxygen is processed by the organisms into the extremely toxic molecules of Singlet Oxygen (1O2), Superoxide Ion (O2-), Hydrogen Peroxide (H2O2), Hydroxyl Ion (OH-), and other toxic molecules.


Facultative anaerobes and obligate aerobes

Facultative Anaerobic Organism s can survive in either oxygenated or deoxygenated environments and can switch between cellular respiration or fermentation, respectively) and '' Obligate (strict) Aerobes '' (organisms that can survive only with oxygen) have special enzymes ( Superoxide Dismutase and Catalase ) that can safely handle these products and transform them into harmless water and diatomic oxygen in the following reactions:

:
''2O2- + 2H+ –superoxide dismutase–> H2O2 (hydrogen peroxide) + O2.''


The hydrogen peroxide produced is then transferred to a second reaction:

:
''2H2O2 –catalase–> 2H2O + O2.''


The oxidative powers of the superoxide ion have now been neutralized. Only facultative anaerobes and obligate aerobes possess the two enzymes necessary to reduce the superoxide.

In organisms which use Glycolysis , the absence of oxygen prevents Pyruvate from being Metabolised to CO2 and Water via the Citric Acid Cycle and the Electron Transport Chain (which relies on O2) does not function. Fermentation does not yield more energy than that already obtained from Glycolysis (2 ATPs) but serves to regenerate NAD+ so glycolysis can continue. Various end products can also be created, such as Lactate or Ethanol .

Fermentation in animals is essential to human life.

In Lactic Acid Fermentation , the following reaction occurs:

1. '' Glycolysis ''
:
''C6H12O6 (glucose) + 2 NAD+ o 2 C3H4O3 ( Pyruvic Acid ) + 2 NADH''


2. '' Lactic Acid creation''
:
'' 2 C3H4O3 (pyruvic acid) + 2 NADH o 2 C3H6O3 (lactic acid) + 2 NAD+''


''Net reaction'':
:
''C6H12O6 (glucose) o 2 C3H6O3 (lactic acid)''



Fermentation in other organisms

In some plant cells and yeasts, fermentation produces CO2 and ethanol. The conversion of Pyruvate to Acetaldehyde generates CO2 and the conversion of acetaldehyde to Ethanol regenerates NAD+ .


ANAEROBIC RESPIRATION IN PROKARYOTES